1. Field of the Invention
The present invention relates to an electron beam lithography method, for drawing and exposing elements that constitute an uneven transfer pattern on a master carrier for magnetic transfer, by irradiating an electron beam on a resist provided on a disk.
The present invention also relates to a patterned master for magnetic transfer, having a transfer pattern that corresponds to an amplitude servo pattern constituted by amplitude servo signals.
Further, the present invention relates to a lithography method, for drawing and exposing elements that constitute the transfer pattern on the patterned master carrier for magnetic transfer, by irradiating an electron beam on a resist provided on a disk.
Still further, the present invention relates to a preformatted magnetic recording medium.
2. Description of the Related Art
Accompanying increases in amounts of data, magnetic recording media, which have high capacity, are inexpensive, and preferably enable readout of necessary portions in a short time, that is, capable of so-called high speed access, are desired. Various types of high density magnetic recording media are known. The data recording regions of the high density magnetic recording media are constituted by narrow tracks. Tracking servo technology plays a large role in enabling magnetic heads to scan the narrow tracks to reproduce signals with high S/N ratios. A sector servo technique is commonly employed to perform tracking servo.
The sector servo technique is a technique for causing magnetic heads to correct their positions. In the sector servo technique, servo data, such as servo signals, track address data signals, and reproduction clock signals, are recorded in servo fields. The servo fields are provided regularly at predetermined angles on data surfaces of magnetic recording media, such as magnetic disk media. Magnetic heads scan the servo fields and read out the servo data, to confirm and correct their positions.
A technique that employs reproduction amplitude data of servo signals is commonly applied to servo signals for track positioning. A common servo pattern comprises servo signals in A, B, C, and D burst portions. Each bit of A burst bit strings and B burst bit strings that constitute the A burst portion and the B burst portion is recorded at positions shifted one half of a track width from the center line of a track. When a reproducing magnetic head passes the servo field, positioning servo is applied such that the reproduction signal amplitude of the A and B burst bit strings are the same.
It is necessary for servo data to be recorded on magnetic recording media as preformatting during production thereof. Presently, preformatting is performed by dedicated servo recording apparatuses. The servo recording apparatuses are equipped with magnetic heads having head widths of approximately 75% of a track pitch, for example. A disk is rotated in a state in which a magnetic head of a servo recording apparatus is in close proximity thereto, and servo signals are recorded by moving the magnetic head from the outer periphery to the inner periphery of the disk every ½ tracks. Therefore, a long amount of time is required to preformat a single disk, which is a problem from the viewpoint or production efficiency.
Meanwhile, a method for accurately and efficiently preformatting magnetic recording media has been proposed in Japanese-Unexamined Patent Publication Nos. 10(1998)-040544 and 10(1998)-269566. This method transfers patterns, which are formed on master carriers and bear servo data, to magnetic recording media by magnetic transfer.
Magnetic transfer employs patterned master carriers, which have transfer patterns constituted by uneven patterns that correspond to data to be transferred, to magnetic recording media (slave media), such as magnetic disk media. The master carriers and slave media are placed in close contact, then transfer magnetic fields are applied thereto. Thereby, magnetic patterns that correspond to data (servo signals, for example) borne by the uneven patterns of the master carriers are magnetically transferred to the slave media. Magnetic transfer is advantageous in that: recording can be performed statically, without changing the relative positions of the master carriers and the slave media; accurate preformatting is enabled; and the amount of time required for recording is extremely short.
As a method to produce master carriers, which are utilized in magnetic transfer, an application of an optical disk stamper production method is being considered (refer to Japanese Unexamined Patent Publication No. 2001-256644, for example). The optical disk stamper production method uses an original disk, having an uneven pattern formed of resist that corresponds to data to be transferred, as a base. During production of the optical disk stamper, a disk (a glass plate, for example) having resist coated thereon is rotated. Data is converted to lengths of pits, and data is written into the resist by emitting laser beams, which are modulated according to the lengths of pits, onto the resist.
It is considered that drawing of the fine patterns onto master carriers for magnetic transfer may also be performed by rotating a disk having resist coated thereon and emitting a laser beam modulated according to data to be transferred, similar to the production method for the optical disk stamper.
However, miniaturization and increase of data capacity are desired in magnetic disk media. If bit lengths or track widths are decreased to accommodate increases in recording density (for example, if bit lengths or track widths become 0.3 μm or less), the decreased sizes approach the drawing limits of laser beams. Therefore, the shapes of the ends of drawn portions become arcuate, causing difficulty in forming rectangular elements of the uneven patterns. The shapes of the elements that constitute the uneven patterns of master carriers, and particularly the shapes of the upper surfaces of the elements, are those of the drawn portions. Therefore, if the ends of the drawn portions are arcuate, the upper surfaces of the protrusions of the uneven patterns on the master carrier substrate become shapes different from rectangles, such as ovals. In these cases, it becomes difficult to form desired magnetic patterns on slave media.
Meanwhile, in the field of semiconductors, patterning is already being performed by utilizing electron beams, which are capable of exposure with smaller diameter spots than laser beams. By utilizing the electron beams, it is becoming possible to perform highly accurate patterning of fine patterns.
In addition, patterned exposure using electron beams has been proposed in Japanese Unexamined Patent Publication No. 2001-110050. The patterned exposure using electron beams has been proposed to produce miniature, light weight and high recording density magnetic patterned media, the realization of which is being anticipated.
Accompanying the narrowing of track widths to accommodate increases in recording capacities of magnetic recording media, the accuracy in forming amplitude servo patterns thereon becomes an important factor, from the viewpoint of securing tracking performance of magnetic heads.
Specifically, fine structural elements of a servo pattern may be drawn on an enlarged recording surface during a design step. In actuality, however, the fine elements are drawn by deflecting and irradiating an electron beam on a resist surface of an original size. Therefore, it is difficult to draw the elements according to the designs thereof.
Patterned master carriers have been produced, based on servo patterns drawn by electron beams. These master carriers have been employed to record amplitude servo patterns onto magnetic recording media as magnetic patterns. When these magnetic recording media are actually loaded into drives to perform recording and reproduction, however, there are cases in which the exhibited tracking performance, that is, heads accurately following tracks, falls short of design parameters.
Basically, if production accuracy of servo patterns is low, positioning servo accuracy is also decreased. There are cases in which heads are unable to scan the positions of designed tracks, and scan outside the tracks. However, in practice, it is impossible to form all of the elements of servo patterns with high accuracy. Therefore, it is necessary to study what degree of accuracy is required to secure tracking performance, and to form servo patterns at a level of accuracy that does not pose actual problems in servo positioning.
It is also necessary to investigate drawing methods for the servo pattern that secures the desired level of accuracy.
Servo signals include, for example: a preamble (synchronization signal), which is recorded across an entire track width; gray codes (track number discriminating signals); and burst signals, which are recorded in halves of the track widths, for positioning heads. Transfer patterns (uneven patterns formed in magnetic material), which are formed on the magnetic transfer master carriers to transfer and record the servo signals correspond to the servo signals. That is, the transfer patterns comprise: first elements, which are protrusions formed across the entire track width; and second elements, which are protrusions formed at positions shifted half a track pitch from the first elements so as to straddle two adjacent tracks (or protrusions which are formed across half the track width). It is necessary to draw each element in resists, which are coated on disks, efficiently and accurately by use of electron beams.
The amplitude servo pattern described above is illustrated in the figures of Japanese Unexamined Patent Publication Nos. 10(1998)-040544 and 10(1998)-269566. As illustrated in the figures, the amplitude servo pattern comprises servo burst signals constituted burst bit strings. The burst bit strings are provided in different tracks and adjacent to each other in the track width direction, with intervals of approximately one track width therebetween. The burst bit strings include those which are formed across substantially the entire track width with the center of the track in the track width direction as their centers, and those which are formed from the central portion of a track to the central portion of an adjacent track so as to straddle the two tracks. An electron beam is deflected to draw burst bit strings on adjacent tracks, and also deflected to draw elements, which are shifted a half a track from other elements. It has been found that accuracy in these deflection operations is important in securing servo positioning.
Particularly in the production of the aforementioned master carrier for magnetic transfer, it is necessary to perform patterning concentrically or in a spiral. Therefore, favorable pattern formation is difficult, in the case that an electron beam lithography method that employs an XY stage, as in the field of semiconductors, is adopted. Accordingly, a lithography method, which is capable of drawing favorable patterns, is desired. In particular, the aforementioned second elements, which are shifted half a track pitch from the tracks (or only formed across half the tracks) cannot be drawn in a manner similar to that for drawing the first elements, and therefore, innovations are required. Accompanying increases in the numbers of tracks (numbers of sectors), the number of elements also becomes enormous. Thus, reductions in drawing times, by improvements in drawing speeds, as well as improvements in the shapes and positional accuracy of drawn elements across the entire surfaces of disks, are desired.